Currently, an increasing number of users are using the new locating services (also referred to as position determination services, or LBS for “Location Based Services”).
Locating has many applications. The user can for example use it to find the nearest hotel or restaurant, obtain tourist information about the place that he is visiting. Worried parents can locate their child at any time. Professionals can locate their vehicles, track personnel, deliveries and detect any problems (for example, the stopping of a merchandise lorry in an unauthorised zone).
Two major families in locating methods are known: the GPS method of locating (“Global Positioning System”) and the GSM method of locating.
The GPS technology has enjoyed remarkable success, in particular thanks to the navigation assistance services for motorists. Conventionally, GPS locating is carried out via the reception, via a GPS positioning system comprising a GPS locating module (also called GPS module in what follows), of signals coming from several satellites located in orbit. The GPS module then decodes these signals in order to determine the geographical position of the system.
The major inconvenience of the aforementioned GPS positioning system resides in the fact that the GPS module is constantly on. As such, the electrical consumption of such a system is not optimal.
Among the various GSM locating techniques, one known technique, called Cell-ID (cell identification), consists in locating a radiocommunication device comprising a radiocommunication module (also called GSM module in what follows), for example a mobile terminal, using the address of a BTS (Base Transceiver Station”) whereon is locked the mobile terminal.
In relation with FIG. 1, a flowchart showing the steps of the Cell-ID technique is presented.
Recall first of all that a GSM radiocommunication network includes a plurality of distinct geographical cells. Each cell of the network comprises a BTS making it possible to emit a BCCH carrier (“Broadcast Control Channel”) (also called radiocommunication carrier in what follows) to the radiocommunication devices located in the cell. Each radiocommunication carrier conveys a plurality of information messages (also called system information messages in what follows) on the access rules to the current cell and to the neighbouring cells (minimum and maximum power level, locating zone number, etc.). These system information messages are of a number of 8 and are, in general, noted SYS_INFO 1 to SYS_INFO 8. These system information messages are well known to those skilled in the art and therefore are not described in detail. However, recall that, among the system information messages conveyed by the carrier, the system information message SYS_INFO 3 includes the identifier of the CID cell (for “Cell Identity”) associated with the carrier.
In a first step E1, the radiocommunication module, included in the radiocommunication device which is placed in a current geographical zone, receives carriers for example from a public land network of a mobile telecommunications operator (for example the GSM network of the operator SFR). More precisely, during this step E1, the radiocommunication module implements a scanning of the radiofrequency spectrum in such a way as to detect radiocommunication carriers.
During a step E2, the radiocommunication module implements a carrier selection algorithm. Conventionally, selecting consists in classifying the carriers detected in the step E1 in a decreasing order of power level, then in selecting the one or those which have a power level greater than the noise level.
During a step E3, the radiocommunication module implements a synchronisation algorithm which makes it possible to determine, from among the carriers selected in the step E2, the carrier whereon it can be synchronised.
During a step E4, the radiocommunication module is synchronised on the carrier determined in the step E3. In other terms, the radiocommunication device locks onto the BTS having emitted the carrier determined in the step E3.
During a step E5, the radiocommunication module reads and decodes all of the system information messages (SYS_INFO 1 to SYS_INFO 8) conveyed by the carrier whereon it is synchronised. As such, at the end of this step E5, the module obtains the identifier of the cell associated with the carrier whereon it is synchronised.
During a step E6, the radiocommunication module sends to a remote locating server the cell identifier obtained in the step E5.
Then, during a step E7, the locating server converts the cell identifier into coordinates of the geographical position.
Finally, during a step E8, the radiocommunication module receives the coordinates of the geographical position sent by the locating server.
The Cell-ID technique (GSM locating technique) represented major progress in the detection of the position of an object or of an individual. However, it has a certain number of disadvantages.
First of all, this known technique has the disadvantage of implementing one or several locating servers which are complex and expensive.
Another disadvantage of this Cell-ID technique, resides in the fact that, in order to obtain position information, it requires the implementation of a full scanning of the radiofrequency spectrum (aforementioned step E1), of a decoding of all of the system information messages (aforementioned step E5), and of a data exchange with a locating server (aforementioned steps E6 and E8). The implementation of such algorithms (scanning, decoding, and data exchanges) is expensive in terms of calculation resources and electrical consumption.